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area which is wholly or partly deserted by the native birds is inhabited later ou 

 by an invasion of individuals of the same species from a more northerly area. 



•' The mode in which the want of sustenance produces migration," writes 

 Professor Newton, •' may best be illustrated by confining ourselves to some of 

 the unquestionably migrant birds of our own northern hemisphere. As food 

 grows scarce toward the end of summer in the most northern limits of the range 

 of a species, the individuals afl'ected thereby seek it elsewhere ; in this way they 

 jiress upon the haunt of other individuals : these in like manner upon that of 

 yet others, and thus 



' The waves behind impel the waves before,' 



until the movement which began in the far north is communicated to (he 

 individuals occupying the extreme southern range of the species at that season ; 

 though, but for such an intrusion, these last might be content to stay some time 

 longer in the enjoyment of their existing quarters " (Bictionari/ of Birds, p. 555). 



While admitting that want of sustenance may prompt the autumn migration 

 in some cases, it may lie doubted whether it is so important a factor as is commonly 

 supposed. It is obviously of advantage to birds to perform the journey while yet 

 the food supply is fairly ample, and before their physical powers become imjiaired 

 with fasting. If we suppose that, in a species of migrant, a certain number of 

 individuals delay the movement until hunger compels their departure, clearly a 

 larger proportion of such individuals will succumb to the hardships of the 

 journey than of other individuals that left the breeding-quarters a short time 

 previously. 



There appears to be some evidence that completion of the moult, or, at any 

 rate, passage through the critical stage of moulting,* and also (in adults) decline 

 of the stimulus of reproduction, are factors ; the precise period of departure being, 

 perhaps, largely determined by a marked fall in temperature. 



The early departure of adult Cuckoos {Cuculus canoruis) is often cited as a 

 s])ecial dilKculty. In this species the cares of family life are foisted upon others ; 

 when, therefore, functional activity of the rejiroductivc apparatus diminishes, this 

 circumstance alone may be sufficient to incite the birds to retreat. 



Turning now to the reverse journey, the very striking instances on record of 

 the selfsame spot being selected for nesting purposes year after year by the same 

 species have been claimed as an indication that birds make the return journey from 

 a desire to reoccupy old quarters. These facts may, however, be interpreted in a 

 different manner — namely, as a proof of the eligibility of the particular locality 

 as a breeding area, and of the particular spot as a nesting site. "When a jiarticular 

 spot has afforded to a pair of birds a secure and convenient situation for the home, 

 it is likely enough that one or both of them will prefer to return to it again the 

 following year from its known suitability ; and we need uot attribute to birds a 

 greater partiality for their old haunts than this. Several circumstances may render 

 it impossibh^ fur more than one (and sometimes for either) of the original pair to 

 reoccupy the same place, such as, for instance, the alliance of one of them with a 

 different mate, or death. Of the individuals that return in spring most will do so 

 for the first time, and their knowledge of the exact locality of their birth can hardly 

 be supposed to be very precise, since they left it at an early age. 



• Ct. Dr. Jonathan Dwight, jun., " The Sequenco of Plumages aud Moults o£ the Passerine Birds of 

 New York " (Jn?w?» -V. Y. AcmI, tici. xiii. p. 120). 



